The invention relates to a mounted assembly for a cycle, more particularly for a bicycle, consisting of a rim and a tyre. The invention is directed also at a rim for a cycle wheel and a tyre for a cycle. The invention is directed more particularly at cycles intended to be used with tyres which have to function without inner tube, of the “tubeless” type.
A conventional single-part rim of the tubeless type, intended in particular for automobile vehicles, consists of various zones; first of all, starting from an outer edge of the rim, there is a rim flange, or wing, whose essential task is to retain the bead of the tyre in the inflated state; then the bead seat, on which rests the bead of the tyre and which ensures clamping thereof, as well as air-tightness; and finally a groove designed to allow mounting of the tyre due to its smaller diameter than that of the flange. In the case of a rim comprising humps, the latter are present between the bead seat and the groove which permits mounting. Humps are projections whose function is to prevent or at least to delay the occurrence of unwedging of the tyre in particular in the case of a drop in pressure.
The conventional process for mounting a tubeless tyre on such a rim comprises various stages, which are as follows; firstly, a part of the first bead is passed over the rim flange and this part is placed in the groove. The rest of the bead may then be passed over the flange thanks to slight ovalisation of the corresponding bead wire of the tyre. The same applies to passing the second bead over the flange of the rim. Mounting is then completed by a final stage involving inflation to a pressure which is such that it ensures positioning of the beads on the seats resting on the rim flanges. During the latter stage, the beads pass over the humps which form an obstacle to passage of said beads as far as their respective seat.
Bicycle tyres exhibit certain peculiarities; in effect, the bead wires are very flexible and deformation of the bottom zone may thus be considerable. This is not without drawback in the case of a tubeless tyre since tightness must be complete between the rim and the tyre and more precisely between the rim and the beads of the tyre.
Although not limited to such applications, the invention will be more particularly described with reference to mounted assemblies intended to be fitted on bicycles for road use. A particular feature of these mounted assemblies is that the operating pressure is of the order of 8 bar.
It is currently known to make mounted assemblies of the tubeless type for bicycles of the all terrain type; the operating pressure of such tyres is of the order of 2 bar. Patent application EP 0 893 280 has already described an improved rim which facilitates the inflation conditions of such a tubeless tyre and which improves the resistance of the tyre when inflated in the event of violent lateral impact. Such a rim is characterised by the presence on the one hand of a narrow groove which promotes the inflation phase by ensuring tightness between the beads and the rim; and on the other hand, this rim is characterised by the presence at the groove edge of humps which permit retention of the beads of the tyre on the seats provided to this end in particular in the event of violent lateral impact, for example in the event of impact against stones.
Use of this type of rim to produce a mounted assembly intended for road use and therefore brought to pressures of the order of 8 bar is impossible. It transpires that, if the use of a rim such as that described by the patent application EP 0 893 280 effectively allows inflation of the tyre to be initiated with certainty, whatever the characteristics of the tyres tested, beyond a certain pressure unwedging of at least one bead occurs with the consequence that the tyre rolls off the rim. Furthermore, the pressures reached at the time of unwedging of the beads of the tyre are generally lower than 8 bar and therefore lower than the operating pressure commonly required and distinctly lower than the pressures which may be reached in extreme situations such as that described above.
The Applicant has already produced rims of another type for production of a mounted assembly intended for road use. Such a cycle wheel rim is characterised by bead seats having, depending on the profile of the rim in a radial plane, a generatrix whose outer end is on a circle of a diameter which is less than the diameter of the circle on which is located the axially inner end, said generatrix forming an angle with the axis of rotation of the tyre of between 15 and 45 degrees.
“Axial” is understood to mean a direction parallel to the axis of rotation of the rim and “radial” to mean a direction intersecting the axis of rotation of the rim and perpendicular thereto. The axis of rotation of the rim is the axis about which the rim turns in normal use. The median circumferential plane is a plane perpendicular to the axis of rotation of the rim and which divides the rim into two halves. A radial plane is a plane which contains the axis of rotation of the rim.
The tests performed with these rims show that it is possible to achieve the operating pressure for road use during mounting without observing any unwedging of the beads.
The improvements made to the profile of the rim thus designed therefore permit the production of a mounted assembly of the tubeless type for road use which guarantees satisfactory inflation.
The rim profile thus defined makes it possible to maintain rim/bead contact at higher pressures than those measured during unwedging of the bead on a known rim.
However, it would appear that where the conditions of use involve subjection of the mounted assembly to extreme conditions, it is possible to observe looseness of the tyre or even unwedging thereof. The pressure may reach values of the order of 11 bar when the tyres are subjected to such extreme conditions, in particular during competitions when the temperature of the internal air of the tyre may increase very substantially, and lead to an increase in pressure. This is the case, for example, during heavy braking when descending hills, when the friction of the brake pad on the rim results in such an increase in temperature.
The inventors thus set about trying to produce a mounted assembly consisting of a rim and of a tyre of the tubeless type which does not present any risks to the user, in particular that of the tyre rolling off the rim, at the pressures conventionally required for this envisaged type of use.